Powder coating compositions for use in the coating or painting of the surfaces of objects are extremely desirable. In typical use, the powder coating compositions are electrostatically applied to the object to be coated. The coated object is then heated, which results in the cure of the powder coating material and in its being bonded to the object. The primary advantage of powder coating compositions over liquid coating systems is that, in the case of the former, no volatile solvent or other liquid vehicle is given off during the curing or drying of the coating. This is particularly significant in large scale commercial coating operations in which it is desirable and necessary to minimize.
Various powder coating compositions are known in the art. Typically, these materials are comprised of minute particles of resinous compounds. These resinous materials are finely pulverized to produce powder particle sizes ranging, for example, from a few microns to perhaps 50 microns.
It has been prior art practice to apply these finely pulverized powder coating materials to an object to be coated by use of electrostatic spray techniques. In a common electrostatic spray installation, the object to be coated is suspended within and moving through a spray booth where it is electrically grounded. An operator standing outside the spray booth holds an electrostatic spray gun to which a high voltage of appropriate polarity is applied. This voltage may be on the order of tens of kilovolts. A mixture of compressed air and the powder coating material is guided through the electrostatic spray gun and is discharged into the booth through an opening provided therefor. A cloud of air-suspended powder is produced within the booth and a small portion of this powder is electrostatically attracted to the surface of the object to be coated. A vacuum source draws the remainder of the air and powder coating mixture from the booth through hoppers in the floor of the booth. This booth exhaust mixture is conveyed to a dust collector where the unused powder coating material is removed from the air. The collected powder coating material may be recycled if desired when a single color is being applied in the booth. However, when powders of different colors are applied within a short time span without completely cleaning the booth and hoppers, the collected powder is a mixture of the colors and must be discarded.
In the case of large objects to be coated, it is possible to construct a large booth wherein the operator actually stands within the booth during the application of the powder coating material. However, it may be appreciated that the operator must be protected from the danger of powder inhalation, and the subatmospheric or vacuum pressure source for removing the air-suspended powder coating material from the booth must be of very large capacity.
The prior art systems described above have a number of disadvantages, several of which are:
1. They do not allow precise, definite control of the spray pattern and, thus, are not ideally suited to form a controlled design or pattern on the object being coated.
2. Where a small booth is used and the operator stands outside, only small objects may be coated.
3. Frequent changes of color are difficult because the booth and any vacuum exhaust from the booth must be thoroughly cleaned with each change of color if the unused powder is to be recycled. Failure to do this could result in a mixture of the various colors used. This problem is particularly acute in the case of a large booth where frequent color changes are even more difficult because of the booth size, a thorough cleaning of the booth being both time consuming and difficult. In those installations where it is necessary to frequently change colors, e.g., painting of automobile bodies with top coat materials, it is not practical to clean the system between colors and, therefore, it is impossible to recycle excess powder. As mentioned above, the excess powder is generally collected in hoppers in the floor of the booth and drawn by vacuum to bag houses. Obviously, when frequent changes of color are affected, the powder being drawn to the bag house contains a mixture of pigments. Thus, in such an operation, the excess powder must be discarded. This is not only wasteful, but also creates a disposal problem.
4. Because of the size of most spray booths relative to the size of the article to be coated and the open nature of the booth, any return system, to be effective, must be inordinately large. This, of course, is inefficient and raises the expense of the installation.
5. Because of the nature of most prior art spray booth installations, i.e., the object to be coated moving in an unconfined manner through the center of the booth on a conveyer, there are serious overspray problems resulting in contamination of the booth and a concomitant wast of powder.
6. Because of the somewhat open nature of the interior of most booths, stray drafts tend to affect the dust clouds causing possible variations in application density and a waste of coating material.
Ser. No. 443,555 filed Feb. 19, 1974 and now abandoned as a Continutation-in-Part of Ser. No. 212,294, and now abandoned, both filed by the present applicant discloses an apparatus and method which overcomes many of the deficiencies discussed above. The apparatus disclosed in that application permits frequent color changes allows more complete recovery and recycling of each color of powder employed, is adaptable for coating large as well as small objects, maximizes use of a vacuum return system, substantially reduces overspray and draft problems, and allows precise, definite control of a spray pattern. The process which achieves those results and which was claimed in that application comprises passing a powder mixture into a shroud having an opening of determined configuration positioned adjacent the grounded object to be coated so that the mixture is confined to a volume bounded in part by the object to be coated. This permits the pressurized mixture dispensed by the electrostatic spray gun to expand within the shroud, filling it with the mixture. A portion of the charged particles are electrostatically attracted to the object to form a coating on it in the configuration of the shroud opening. By employing a shroud with a configured opening it is possible using that method to apply a sharply defined pattern of paint, an accomplishment which theretofore had not been possible in the application of powders and possible in the application of liquid paints only with the use of masking. After application of the coating to the article, the remainder of the expanded mixture in accordance with that process is drawn with a subatmospheric pressure source into a chamber positioned around or surrounding the shroud. This chamber is wholly external of the interior of the shroud and after the powder enters the chamber, the mixture flows toward the subatmospheric pressure source.
While the above discussed method and the apparatus for carrying it out overcomes many of the deficiencies of the prior art powder coating techniques and apparatus, it is still not ideally suited for application of powder coatings to substrates, particularly those of a curved contour. In fact, a serious disadvantage of the method is its marginal performance in coating sharply curved surfaces due to its dependence on a flat surface for forming a sufficient draught of air capable of causing atmospheric air to move unadhered powder to the powder collector. Also, that process, although a great improvement over prior art techniques, still demonstrates some deficiencies with respect to powder return for recycling. For maximum results with that method, it is necessary to space the shroud of the apparatus a maximum of 1/4 inch from the surface to be coated in order to effect efficient return of the powder. Also, without a flat surface in place, the device used in that process is not capable of recovering powder completely. Still further, the method and device of the former application does not demonstrate optimal efficiency insofar as application of the powder to the substrate is concerned. In fact, adhesion of desposited powder with that technique is weaker than desirable. Finally, the device and method of the prior application, although an improvement over the prior art, still requires more care in cleaning than is desirable to change colors rapidly.
It is the object of this invention to overcome each of the above deficiencies by providing an apparatus in which: (1) powder over-spray is eliminated nearly completely due to a unique method of vacuuming electrostatically unadhered powder; (2) controllable auxiliary air, fed from a manifold to a mixing chamber, maybe used to assist in purging for color change; (3) application efficiency of virtually 100% is achieved due to recycling of unadhered powder; (4) precise spray patterns are achieved, controlled by the configuration of the applicator opening; (5) the device is adaptable for automation; and (6) the device demonstrates an outstanding ability to coat curved surfaces.